There are various kinds of synthetic resin containers, and they are designed according to their intended use. Various methods for molding these containers are also suggested: a method in which the bottom of an extruded parison is pinched off for air blowing so as to prepare a hollow molded article such as a bottle; a method in which an injection molded parison with a bottom is directly air blown to obtain a hollow molded article; a method in which an injection molded or an extruded parison is temperature adjusted and axially stretched thereafter for air blowing; and so on.
In any of these methods, the bottom of a synthetic resin container can easily break due to irregular thickness distribution. In considering a container for a material under pressure such as carbonated water, the container should not break nor deform anywhere when sealed and under an internal pressure of 3.5 kg and a temperature of 55.degree. C. The container should not deform, and should be shaped so that cleaning of the interior is easy.
It is considered difficult, in general, to mold a container of a thermoplastic synthetic resin without adopting a special shape and still satisfy these conditions. A container which is improved with respect to strength and which is manufactured by the stretching and blow molding method cannot achieve this due to deformation at the bottom.
This bottom deformation is due to the fact that the thickness of the bottom is as small as 0.2-0.5 mm, and the bottom is flexible although it is rigid to a certain extent. The bottom protrudes internally for better stability but is pressed outward and expanded by the internal pressure. However, a synthetic resin container is convenient for carbonated water due to its light weight and ruggedness. Currently, the bottom is often expanded radially outward so that its rounded bottom will no longer be deformed by the internal pressure. Then, since the container cannot stand as it is, a base cup of a synthetic resin is attached to the bottom.
This attachment of the base cup manufactured separately is performed by painting an adhesive to the bottom of a container obtained by blow molding. Pressure is exerted while the adhesive hardens. If the base cup is attached without exerting pressure, the container floats on the adhesive and thickness of the adhesive layer becomes irregular, resulting in variations in the height of the containers. Although this height difference is small, problems arise when the containers are put on the filling line. This is undesirable in a system wherein the molding and filling of containers are performed in a continuous manner. Thus, the adhesion often had to be performed manually, and a special adhering device was required for this.
For attachment of the base cup without using an adhesive, the known insert method may be utilized. However, it is considered practically impossible, due to the position of the blow mold, to set a base cup at the bottom mold of the blow mold of a molding machine which performs injection molding and blow molding of a parison. Thus, a special device is generally required to set a base cup at the bottom mold within the series of molding cycles.